1. Field of the Invention
The present invention relates to a retrofocus type wide angle lens and, more particularly, to a retrofocus type lens capable of close distance correction.
2. Related Background Art
A focusing method of a retrofocus type lens is often achieved by entire extension for integrally extending an entire lens system. However, in a lens system which is entirely moved upon focusing, a total weight of movable groups including a lens barrel holding the lens system is increased. In particular, in an ultra-wide angle lens system or a lens system having a very long back focus as compared to a focal length, a negative refracting power of a front group is increased, and aberration correction cannot be performed. In order to prevent this, the number of lenses is increased, and a front-element diameter is normally increased, resulting in large total length and weight.
However, in a lens system for a camera incorporating an auto-focus mechanism, a response time must be short, a movable section must be compact and light in weight, and a moving distance thereof must be small. Moreover, a retrofocus type lens system of a two-group structure has strong asymmetrical characteristics. Therefore, when an object distance is changed and a photographing magnification is changed upon focusing, various aberrations, in particular, astigmatism, curvature of field, and coma, are considerably changed as compared to other aberrations. For this reason, various efforts have been conventionally made to render a lens system compact and to improve operability while performing aberration correction by means of the following prior art.
A first prior art is a method for relatively moving a given lens group while performing entire extension in a retrofocus type lens system. For example, the first prior art is disclosed in British Patent No. 1,238,668.
As disclosed in Japanese Patent Laid-Open (Kokai) No 140910/1986, a second prior art is a method wherein two lenses located at an object side are fixed, and all other lens groups located at an image side are moved to perform focusing.
As disclosed in U.S. Patent Publication No. 4,311,367 and Japanese Patent Laid-Open No. 202414/1983, a third prior art is a rear-group extension method wherein only a lens group closest to an image side is moved to perform focusing.
However, the above-mentioned prior art has the following drawbacks, respectively.
In the method of the first prior art for moving a given lens group while performing entire extension to perform close distance correction, the weight of movable groups including a lens barrel is very large as in the entire extension method, and the total length of the lens system is increased. For example, in entire extension type focusing in British Patent No. 1,238,688, an ultra-wide angle lens system or a lens system having a very long back focus as compared to a focal length has a strong refracting power of a front group, and aberration correction cannot be performed. In order to prevent this, the number of lenses is increased, and a diameter of lenses of the front group is increased, resulting in large total length and weight. Therefore, in the method wherein entire extension is performed while moving a given lens group to perform close distance correction, the lens system is large and heavy, resulting in poor operability.
In the method of the second prior art, disclosed in Japanese Patent Laid-Open (Kokai) No. 140910/1986, wherein two of three front-group lenses located at an object side are fixed, and all other lens groups closer to an image side than the two fixed lenses are moved to perform focusing, focusing groups to be moved have a large size and become heavy. The lens system is inconvenient for a lens system for a camera incorporating an auto-focus mechanism in view of operability. In addition, the method of correcting various aberrations disclosed in this patent is insufficient. In an ultra-wide angle lens having a large field angle, in particular, changes in coma and curvature of field in a close distance photographing mode cannot be sufficiently corrected.
In the rear-group extension method of the third prior art, disclosed in Japanese Patent Laid-Open No. 143517/1980, (U.S. Pat. No. 4,311,367) wherein a lens group closest to the image side is moved to perform focusing, a movable lens group is compact and light in weight, and is suitable for a lens system for a camera incorporating an auto-focus mechanism. However, in a close distance photographing mode, changes in various aberrations, in particular, coma are considerable. Furthermore, in the method disclosed in Japanese Patent Laid-Open (Kokai) No. 202414/1983, the weight of the focusing group can be reduced, and the number of lens can also be decreased, resulting in good operability. However, changes in various aberrations, in particular, coma in a close distance photographing mode cannot be sufficiently corrected as in the lens system disclosed in Japanese Patent Laid-Open No. 143517/1980.